The instant invention pertains to polyfunctional phenolic hydroxyl terminated hardener products and to curable solid epoxy resin compositions containing said hardeners.
The reaction of epoxy resins with phenolic hydroxy-containing compounds in the presence of a catalyst is well known in the art. This reaction is typified by the reaction of bisphenol A (=BPA or 4,4'-isopropylidenediphenol) with liquid BPA epoxy resins to form solid high molecular weight products.
U.S. Pat. No. 3,931,109 teaches the termination of basic liquid BPA epoxy resins as well as epoxy novolac resins with bisphenol A to give phenolic hydroxyl terminated hardeners.
U.S. Pat. No. 3,931,109 also teaches curable compositions of liquid epoxy resins containing a dispersion of a solid phenolic hydroxyl terminated hardener therein.
The instant polyfunctional phenolic hydroxyl terminated hardener products are superior to the hardeners described in U.S. Pat. No. 3,931,109 in respect to curing solid epoxy resins to cured products with outstanding properties particularly in regards to chemical resistance. This is especially of value in the field of coatings.
The products of this invention are useful as curing agents for epoxy resins. When these items are combined with di- and polyepoxide resins and cured at elevated temperatures (e.g. 180.degree. C.) a dense crosslinked network is established which produces superior protective coatings. These coatings, when applied by electrostatic techniques and properly cured to produce thin films, excel in chemical resistance while maintaining flexibility.
The reason these multifunctional hardeners produce their extremely good chemical resistance is through formation of a dense crosslinked reticulum in the final cured coating. The difunctional BPA terminated hardeners form linear extensions, while the multifunctional BPA terminated curing agents produce a web-like structure. This three dimensional network provides the tight barrier by which the cured final coating strongly resists any chemical attack.
The reaction of hydroxyl groups with liquid epoxy resins to form higher molecular weight epoxy resins is described by H. Lee and K. Neville, "Handbook of Epoxy Resins", McGraw Hill, 1967, New York, pp. 2-6 , 2-9. Chapter 2 of Lee and Neville's "Handbook of Epoxy Resins" is devoted to the "Synthesis of Glycidyl-Type Epoxy Resins". This chapter describes the synthesis of high molecular weight epoxy resins based on many types of alcohols.
U.S. Pat. No. 4,322,456 discloses powder coating compositions consisting of an epoxy resin, a phenolic hardener and a catalyst for effecting reaction between said resin and said hardener.
The instant polyfunctional phenolic hydroxyl terminated hardener products are superior to the hardeners described in U.S. Pat. No. 4,322,456 in respect to curing solid epoxy resins to cured products with outstanding properties particularly in respect to chemical resistance while maintaining flexibility.
U.S. Pat. No. 4,288,565 pertains to epoxy molding compositions wherein triphenols such as 1,1,3-tris(4-hydroxyphenyl)propane are used as the phenolic hardener component. While both the instant hardeners and those of this reference have three or more phenolic groups per molecule, the remainder of the respective hardener molecules differ widely. The prior art hardener is a relatively small molecule with the terminal phenolic hydroxy groups attached to the backbone trimethylene-(=propane) chain at very close intervals. While this does not interfere with the hardening efficacy of the molecule, the cured epoxy resin is a relatively rigid and inflexible structure due to the high crosslink density (nearness together of the original phenolic hydroxyl groups).
By contrast the instant hardeners, wherein the terminal phenolic hydroxyl groups are relatively far removed from one another with a relatively bulky linking group therebetween, permit good curing since the efficacy of the phenolic hydroxyl groups is undiminished. However, a lower crosslink density is obtained leading to a concomitant increase in flexibility without loss of chemical resistance.
The preponderance of hydroxyl groups present in the instant hardeners also allows for improved adhesion of epxoy resin cured thereby.